“Incidentally, this isn’t something that we widely publish, but the fact is that the paintings at the National Gallery are not insured.

“Because we couldn’t possibly afford to insure them. They are priceless. How could you possibly insure even just The Ambassadors?”

She added of art galleries in general: “Nothing is insured in this country. You couldn't possibly afford to insure them. The moment they go off-site on tour then they become insured.

“They're too valuable, they’re priceless. Shocking but true.”

Room attendants are “extremely highly trained” in protecting the works, including on “how to intercept lunatics”Credit:
UniversalImagesGroup

On the question of what would happen if someone attempted to damage a painting, or there was a fire, she conceded: “Yeah, you're in trouble.

“The room attendants are extremely highly trained in all sorts of things including how to intercept lunatics. And there are CCTV cameras and everything else.

“But yeah, it's really vulnerable.”

“Very recently, somebody did have a go at a picture and I think what happened was the attendant went and launched themselves at them and so did members of the public.

“I think it was an instinctive reaction. You wouldn’t say to somebody, put yourself in danger. But actually instinctively what happens is that the room wardens get so protective over paintings, and the members of public love them so much, that they don’t want members of the public attacking them.

“The public sometimes are the greatest defenders of the work of art. It’s happened twice now it's been the public which has protected a work of art.

Rothschild, an author and philanthropist, said the public had twice protected the art work from vandals in recent timesCredit:
View Pictures/UIG via Getty Images

Speaking to Sir David during a later public talk, Sir Nicholas Serota was asked the same question.

"The only things that are insured at the Tate are the works that are being lent to us,” he said.

"We're not allowed to insure because the cost to the Exchequer would be huge. And I think there's every confidence in our security.”

He added: “Which is not to say that nothing has ever disappeared from the Tate and not to say nothing has never been introduced to the Tate.”

A spokesman for the National Gallery said: "The National Gallery takes every precaution to ensure the safety of its Collection, its visitors and staff.

"However, we never discuss our security measures in detail as to do so could compromise our security.

"The Gallery’s collection, when it is displayed on site, is not commercially insured consistent with the principle that Government property (including the whole of the British National Collection) is self-insured.

"When we lend works we require borrowers to insure the works to their full value. Also, when the Gallery borrows from third parties the works we borrow are insured under the UK Government Indemnity scheme."